This invention is in the field of active filters in electronic circuits. Wireless communications and electronics can benefit by improved circuits, devices, and systems for wireless communication, and processes of operating and making them.
In narrowband filter design, a tuned LC filter is used to provide selectivity on receive and provide filtering for transmit signal paths. An approximate frequency called a 3 dB point at which impedance falls 3 dB from a peak impedance value of the LC filter is given by a bandwidth Fo (1−1/(2Q) where Q is the quality factor.
For an integrated LC filter at 2.4 Ghz with a Q=8, 3 dB filtering will happen near 2.25 GHz. For close-in interferers (10 MHz or less to 50 MHz range), the attenuation provided is quite minimal and problematic. In the receive chain, such minimal attenuation increases the linearity requirements for blocks following low noise amplifier LNA, and imposes tighter LO phase noise requirements to handle close-in interferers (reducing in-band noise due to reciprocal mixing). Furthermore, in the transmit chain, such minimal filter attenuation imposes tighter transmitter TX local oscillator LO and signal close-in emission constraints since the filtering provided by the LC filter is minimal at nearby (<50 MHz) frequency offsets from the transmit carrier.
Another on-chip RF (radio frequency for transmit or receive) filter approach operates an LNA. In parallel, a down conversion to baseband feeds a high pass filter the output of which is converted and subtracted from the output of the LNA. An out of band interference signal called a blocker is supposed to be canceled. In this way an auxiliary path with frequency translation and a summer is supposed to create a narrowband RF filter response. That approach introduces the expense of auxiliary path mixer circuits to handle the interferer, and the additional hardware and extra noise introduced by the auxiliary path may not be insignificant.
Yet another RF filter approach is disclosed in the U.S. Pat. No. 6,597,900. A forward path and feedback path with frequency translation are used to offer a controlled impedance at the input. This is supposed to be used along with RF filters to achieve narrowband tuning. The expense of additional hardware including a mixer with high reverse isolation may be not-insignificant.
Another approach might consider a Q-multiplier to increase the Q by an amplifier with output and input coupled to LC, i.e. positive feedback not reaching oscillation. Unfortunately, to get a 3 dB filtering at a nearby frequency (e.g., 5 MHz away from RF signal frequency of 2.4 GHz), the Q required can be on the order of 250. Starting from a Q on the order of 10 for on-chip LC, it is believed not practical or economical to achieve this reliably.
It would be desirable in the art to somehow find a less expensive, less constraining, and less noisy way of doing RF filtering reliably, especially for integrated circuit RF filtering.
It would be desirable to address the above mentioned problems and issues, among others.
Mobile telephony can communicate video and digital data, and voice over packet (VoP or VoIP), in addition to cellular voice. Streams of information such as video, voice, audio content, images of all kinds, and data should be flexibly handled by such mobile devices and platforms. Wireless local area network (WLAN) systems transmit and receive some wireless signals. Short range types of wireless called Bluetooth and Zigbee also exist. Solving problems in integrated circuit RF filtering can benefit numerous forms of wireless and other electronic products and applications.